[BMWi8]

This thread will review upgrades to the Avigo (Aria Child) BMW i8 6V electric powered car. Note that a 12V version does exist, but I am not familiar with it.

PHASE 1 - BATTERY RUN TIME UPGRADE
The first thing I noticed was insufficient "run-time". After about 20 minutes of use, the i8 sounded different and was visually slower. Sure enough, checking the LED "power meter" showed only the red LED illuminated. There are 2 main factors which affect this run-time, that being load on the motor and battery capacity (amp hour rating). I cannot easily change the load on the motor, as this is determined by weight of the operator and resistance (both slope and rolling resistance). I can however change the battery to a higher Ah rating!

The battery tray appears to be designed for several different sizes. After taking some measurements I went with the 6VDC 12Ah sealed lead-acid battery by Keyko Japan.

Supplies
1. Wire stripper & terminal crimp tool
2. 2x 1/4" female spade connectors (for 12 gauge wire)
3. 6VDC 12Ah replacement battery by Keyko
4. Phillips screwdriver
5. Rubber bumpers
6. Electrical tape (optional)

Procedure:
1. Remove original battery, 2x phillips screws and black battery cap
2. Cut off red (positive) and black (negative) terminals as close as possible to their ends
3. Strip 1/4" of insulation off each wire and crimp on 1/4" female spade connectors
4. Add rubber bumpers as needed to reduce battery vibration (vibration can shorten battery life). I did the underside of the hold down bar and one end of the battery
5. Install, cover terminals with electrical tape, charge and enjoy over 50% more runtime!

PHASE 2 will be upgrading from 1WD (the factory installed motor drives only the right rear wheel) to 2WD.

[BMWi8]

PHASE 2 - UPGRADE FROM 1WD TO 2WD
If the BMW i8 attempts to climb the valley between the street and driveway at an angle, the single drive wheel can loose traction and the car gets "stuck". The ability to go through grass is a struggle and use in light snow or ice is non-existant. Time to upgrade to 2WD!

You can purchase replacement motors ($15) from http://rollplay.net/products/view/6v-bmw-i8-silver
I chose to wire the 2nd motor in parallel with the existing motor. As such, the 6VDC battery will see half the resistance and output twice the current. There is risk the relays (HF3FF) on the original control board will not be able to handle this additional current, and overheat. The relay data sheet says version "1C" is rated to 10 Amps at 28VDC or 280W. If I divide 280W by the battery voltage of 6 VDC that gives me almost 47 amps... using a DC clamp meter I did record a very brief spike of up to 36 amps from the battery when stalling both motors, so maybe it will be fine?

Supplies
1. Wire stripper & terminal crimp tool
2. male and female connectors and / or solder
3. stranded wire
4. replacement drive motor and gearbox
5. Pry tool & needle nose pliers

Procedure:
1. Cut the majority of wire harness off from new motor, strip and crimp connectors to wire ends
2. Pry off center BMW cap, use pliers to remove cotter pin, remove outer washer and wheel
3. Remove black 1.5" long plastic spacer, ensure inner washer is in place, install new motor and gearbox assembly, re-install wheel, outer washer and cotter pin
4. Use several zip ties (or one long one) to secure gearbox in same manner as original gearbox
5. Depress metal tabs inside original motor harness connector, remove both contacts, solder stranded wire and re-insert contacts
5. Connect wires, noting that the white wire from the new motor needs to connect to the blue wire of the original motor to allow both motors to operate in the same direction (otherwise you'll spin in a circle)

Observations:
The extra torque handles "heavier" riders easier. Runs through the grass better. Does make a louder noise during turns, as the outside wheel needs to spin faster than the inside... which the motors aren't set up to do.

PHASE 3 will be upgrading to a 12 VDC ESC system... in about a month

[Masterjosh]

was just curious if u had any update on your project. I have a 6v Rollplay chevy police truck and the inside guts looks insanely close. everything looks identical, so i went on a whim and just ordered the gearbox as i believe its prob the same, however its odd that on the parts page for the truck my son has it doesn't list the motor/gearbox as a part you can purchase..so i went to the 6v truck non police version and it was there. also curious in what the difference would be if u wired the motors in series instead of parallel as you did? was just curious how things were holding up for you.

[vtjballeng]

PHASE 2 - UPGRADE FROM 1WD TO 2WD

PHASE 3 will be upgrading to a 12 VDC ESC system... in about a month

So, how has the durability been? Did you upgrade to 12V?

[BMWi8]

was just curious if u had any update on your project. I have a 6v Rollplay chevy police truck and the inside guts looks insanely close. everything looks identical, so i went on a whim and just ordered the gearbox as i believe its prob the same, however its odd that on the parts page for the truck my son has it doesn't list the motor/gearbox as a part you can purchase..so i went to the 6v truck non police version and it was there. also curious in what the difference would be if u wired the motors in series instead of parallel as you did? was just curious how things were holding up for you.

In series would result in each motor seeing half the voltage... so top speed would be almost half, but you would have 2WD and more rimpull or torque to drive in rougher ground (higher rolling resistance) like grass. So far, no issues with the extra current going through the relays... but I didn't run more than 2 months and removed the factory relay board to run a 12 VDC setup through my own relay.

[BMWi8]

PHASE 2 - UPGRADE FROM 1WD TO 2WD

PHASE 3 will be upgrading to a 12 VDC ESC system... in about a month

So, how has the durability been? Did you upgrade to 12V?

No issues for the 2 months running higher current through the factory relay board. Yes, I upgraded to 12 VDC. Hope to post results this month!

[BMWi8]

PHASE 3 - PART 1 - Adding 12 VDC battery
Since the dashboard, steering wheel sounds, lights and aux audio input with amplifier all rely on 6 VDC, I decided to leave the 6 VDC battery and add a 12 VDC battery. I went a bit overkill with 18Ah, which adds extra weight, but I suspect it will take at least a hour of operation to run this battery down to where you would notice lower power.

Supplies:
1. 12 VDC AGM battery, 18Ah or less
2. 1/2" by 1/2" aluminum angle (3 foot long pieces are at most hardware stores)
3. 1/8" aluminum pop rivets
4. 2x 3/16" carriage bolts + 2x nuts

Procedure:
1. Measure and cut the aluminum angle per the dimensions of your battery, allowing extra length to rivet the ends
2. Drill holes in each "corner" and rivet to make a 4-sided aluminum frame
3. Drill 2x 3/16" holes mid-way on the left and right side of the frame, along with matching holes in the plastic car body
4. Trim out the car body holes to a square shape to match the square underside of the carriage bolt
5. Install battery, frame, bolts and tighten down nuts

Observations:
The extra weight at the rear may help traction, but the car is heavier to pick up and move around. The last photo shows the black battery mounted against the back wall at an angle. This is a sealed AGM battery, so the angle won't affect operation or battery life. You need a battery shape that doesn't stick out far enough to interfere with installation of the seat - keep this in mind as some battery designs may be too "thick".

... next will be the ESC and new foot throttle to make use of the 12 VDC battery (you can see the silver cased ESC in the last photo)

[BMWi8]

PHASE 3 - PART 2 - FOOT PEDAL REPLACEMENT
I went with an $8 Foot Pedal I found on eBay. It uses a 49E Hall Effect semiconductor which according to a generic datasheet allows 3 to 6.5 VDC input and varies the voltage output from 0.86 - 4.21 VDC based on the strength of the magnetic field it sees. When I applied 4.5 VDC to the pedal, the maximum output voltage (pedal fully depressed) was about 3.8 VDC on the green wire. When I applied 6.0 VDC, the maximum output voltage (pedal fully depressed) was about 5.2 VDC on the green wire... which is greater than what the spec sheet indicates. This should work well with a ESC designed to be controlled by a hall effect sensor and eliminate the "whiplash" from the factory's almost instant on design.

UPDATE 2/7: My notes were off on how I attached the spacer plate, corrected below plus a photo of the 49E sensor
UPDATE 2/21: The glue failed and the plastic ABS spacer separated from the car body. See 2nd observation section below.

Supplies:
1. Foot Pedal w/ integrated hall effect sensor (eBay search for "Foot Pedal Electric Car Vehicle Throttle")
2. 1/8" thick ABS plastic sheet
3. Plastic adhesive
4. #10 x 1" pan head sheet metal screw

Procedure:
1. Pry out old pedal and note plastic car body has a slight depression about 1/8" deep
2. Trace old pedal (after removing the switch) on 1/8" ABS sheet and cutout a spacer plate in the same shape
3. New pedal has 3 tabs which stick out from the bottom. Cut 1 notch and 2 slits in the ABS plate for these 3 tabs, then make a center cutout to allow the pedal to be fully depressed
4. Use plastic adhesive to secure the spacer plate into the depression
5. Remove the two top screws and drill a single hole down through the BMW floor. A #10 x 1" sheet metal screw was used to secure the new pedal.

Observations:
Fabricating the adapter plate was time consuming, but the fit is solid. I applied some plastic adhesive to help keep it in place, but this may not be the best permanent solution.
I noticed the pedal was difficult to depress all the way down. The rubber boot doesn't collapse very well, so I removed the 2 top screws, swung open the pedal and cut off the top 1/2 of the rubber boot. It may not protect the hall sensor from water as well, but you can fully depress the pedal much easier. All the attached photos show the rubber boot removed for clarity.

Observations update 2/21: the plastic adhesive failed so I fabricated a rectangular plate from the remaining ABS plastic sheet. I slide this into the left and right slots from the old foot throttle. I also cut a piece out in the middle to allow pedal movement... so it's more of a wide U shape now. The single #10 x 1" sheet metal screw now goes into both ABS pieces and locks the new foot pedal in place. See the 7th pic.
I also removed the rubber boot completely from the foot pedal. It would only make pedal travel more difficult.

Next will be the ESC to make use of the new foot pedal...

[BMWi8]

PHASE 3 - PART 3 - ESC (Electronic Speed Controller) UPGRADE

One negative of the factory circuit board is it applies full battery voltage instantly to the motor (or motors if you did the 2WD upgrade). This sudden application of torque tends to jolt the operator (a bit of whiplash). This occurs in forward and reverse. I figured this would only get worse moving up to 12VDC and it's not good for tire life or starting out in slippery ground conditions. I wanted to have variable speed... so enter the foot throttle with hall effect sensor (previous post) and ESC below.

I went with an $9 ESC rated to 250W I found on eBay. It uses a LM339-N Square Wave Generator by TI. I've attached a photo of the circuit board for those curious types. You apply 12 VDC to the thick red wire and ground to the thick black wire. Then attach your hall sensor (foot pedal in this case) to the 3-wire connector - this ESC supplies +5 VDC to the red wire, Ground to the black wire and then looks for a variable voltage (0-5 VDC) on the yellow wire. If the ESC sees no voltage on the yellow wire, it doesn't send any signal to your motors. Partially depress the foot pedal, send say 3 VDC to the yellow wire and the ESC will send a square wave to your motors, but the square wave will have a LOW duty cycle. Fully depress the foot pedal, send 5 VDC to the yellow wire and the ESC will send a square wave to your motors with a HIGH duty cycle. The thick blue and yellow wires go to the motors. I haven't made use of the blue & black "brake" wires yet.

Supplies:
1. 12VDC 250W ESC
2. Multimeter
3. 6 VDC SPDT relay rated at 30 amps (TE Connectivity 1432873-1)
4. Various wire and 1/4" insulated crimp female spade connectors

Procedure:
1. Cut the 2 wires which supply power to the headlights and wire them with all the other LEDS - see first 2 photos. These headlights were designed to only see the 6 VDC applied to the motors and I am moving to 12 VDC. Now the headlights will be on anytime the ignition switch is turned on.
2. Cut the connectors off the wires running to the factory relay board, remove original factory relay board and throw in garbage, connect Orange wire to +6 VDC battery, save Blue and Yellow wires for a later date. White and Black are not used (I connected the Pink wire to the dashboard wiring Ground).
3. Mount 6 VDC relay to factory relay board location, attach factory Red wire to relay Term 86, run a wire from the 6V battery Ground terminal to relay Term 85 and test that the i8 start button now makes the relay click on and off.
4. Run wire from 12 V battery Ground to ESC thick black wire. Run wire from 12 V battery + terminal to relay Term 30, attach ESC thick Red wire to relay Term 87 and test that the relay now turns power on and off to the ESC.
5. Connect the 3 wires from the foot pedal to the 3 corresponding wires on the ESC (red to red, black to black, and green to yellow). Depress the foot pedal and watch voltage vary from 0 - 5 VDC on the green/yellow wire.
6. Connect the motors to the thick blue and yellow ESC wires. Put the car in the air and test for wheel speed based on throttle input! If the wheels go in the wrong direction, just swap the thick blue and yellow wires.

Observations:
I highly recommend bench testing your wiring with a multimeter before sticking an operator in the seat.
After my first couple runs I noticed the ESC case was very hot to the touch. I thought about wiring a 12 VDC fan to blow air over the aluminum case... but then it failed by day 2 (it turns out some of the traces on the ESC circuit board were too thin and melted). It was outputing 12 VDC on the red wire going to the foot pedal (not 5 VDC like it should). The foot pedal didn't like this and wouldn't send a proper voltage to the green/yellow wire. I went back and forth with the ESC seller, but our language miss match didn't get me anywhere. I ordered an identical ESC from a different seller and it runs much cooler - no fan needed.
The ability for the driver to vary ground speed makes the car much more controllable. Top ground speed is close to double and even the neighborhood 8 year olds want to drive it (if they fit)!

This set-up does NOT use the factory direction lever, so you only have Forward direction... I will add a DPDT relay for Reverse capability in Part 4...

[Hammer-fm]

Thanks for sharing and posting that picture of the circuit board. I have been curious about what the low-cost ebay units look like inside and what components they're using relative to the ones I've been building myself. Looks like an ST-micro STP60NF06 FET (the TO220 package on the right), with a totem-pole transistor drive stage (the 2n5401/2n5551 parts). I couldn't find anything on the diode (TO-220 on the left), likely a standard 20A Schottky diode though.

That FET is rated at 42A (at 100C), 14mohm resistance. Hopefully the design is using some sort of current protection because a typical pair of motors will pull 80+A at startup.

My stripboard designs uses multiple (3+) IRFB4110s, rated at 120A each, 4mohm resistance each (360A/1.3mohm total with 3 of them). The first one I built (in the truck) uses a totem-pole driver as well, with a different quad-opamp. The second one I built uses a "real" FET driver IC which seems to work quite a bit better (stronger gate drive = less time spent switching).

Can you get a top-down or higher-res picture of the board? Now I'm curious how much rework would be required to get it up-to-snuff. It's amazing what can be built for $9, and if a replacement FET and some wiring improvements safely brought it up to a 50-60A current limit design it may be worth it. Though to be fair, half of the time I've spent on the stripboard ones is because of soft-start and variable speed limiting, which probably aren't present here.

[BMWi8]

I do not have experience in circuit design, just a decent Fluke multi-meter and a Velleman USB DSO. So you are above my league when talking about improving the $9 ESC with some better components!

I did pull out the first ESC from the trash, so you had good timing. TO-220 on the left is a SBD20C / 100F / SL4M4NB by SL?. TO-220 on the right is a P65NF06 / HW22A 6 / KOR 408 by ST-micro. Diode on the far left is a 1N007S / JX.

I've attached some more direct photos. I'm limited in file size, but you can see the two traces on the back side I repaired. I think one of the glass diodes has failed, near the pair of 47 uF caps.

I'd offer to send you my broken one, but shipping would be about the same as a new one from China.

[Hammer-fm]

Thanks for those shots -- that helps clarify the design. M+ is hooked directly to B+ in this type of design (similar to what I do), and you adjust the speed by varying how much of the time the FET is on (the FET connects M- to B- when it is turned on). As far as the two broken traces you repaired: The upper one is the return path for the diode from M- to B+, which is used when the FET turns off. This is important because the motor is inductive and the current cannot change instantly on inductors -- so it will raise the voltage on the "-" side (M-) until it has a path to somewhere. With the diode, it just feeds back into the battery. Without, it will raise until the FET goes into avalanche, and likely it's a large enough inductance in the motor that it will blow up the FET at that point. Since this trace conducts up to the maximum amount of current the motors can use, it seems very undersized (equivalent to the current limit, if one is implemented, or the stall current if not -- stall current could easily be 80+A for two 12V motors in parallel). The diode itself (20-amp continuous rating) can handle 100+ amps repeatedly as long as they're short duration, but that tiny trace isn't going to last too long even at 20A.

The second/lower trace is the path from the FET drain back to B-, so it's used anytime the FET is on. That's basically carrying all of the current of the motor when the FET is on, so it's under a similar load. I'd also say that seems undersized.

It's not surprising either of those failed; however, it's unlikely that the lower one failed first, because if it fails, the things just stops working but doesn't blow anything else up. If the first/upper one (from the SBD20C diode) fails, it will likely blow up the FET, which will then go to full conduction (most likely). In the process maybe it took a few other things out -- hard to know for sure.

It's very interesting that they went to the effort of solder-coating the nice wide traces at the top (M- to FET source AND part of the trace for the diode return to B+, but didn't bother solder-coating the narrow pinch section, which is where things will fail! If I do get any ESCs on ebay I'll be checking those PCB connections carefully. Each of my FETs and diodes get connected using 18ga solid copper wire (there's no way the stripboard will carry more than 0.5A or so on a trace), equivalent to about about a 5/8" wide trace on a high-quality (2oz copper) PCB.

Is your new one identical inside or a different design (or better put together...). I was also surprised at how many of the components (small caps like the "221" one, the zener diodes (the glass ones you were talking about), and the 2n* transistors) were "floating" on long leads. Normally you'd place these as closely to the PCB as possible. I wonder if a lot of the Chinese sellers get them as bare PCBs and then populate them with (various quality) components.



Again, I appreciate the pictures and thanks for letting me hijack your thread just a bit